A previous known drive apparatus converts rotational motion of a drive source into linear reciprocating motion of a control shaft member through a drive cam and adjusts a controlled variable of a controlled subject in response to an axial position of the control shaft member. In this drive apparatus, when a drive force of the drive source is stopped, it is required to cut a reverse input from the controlled subject to the control shaft member and to hold a given position of the drive cam and a given position of the control shaft member.
For example, in an actuator, which is recited in JP4025155B2, when a nut of a linear motion converting mechanism is rotated through rotation of an electric motor, a shaft portion, to which the nut is threadably engaged, is linearly moved. In a case where a lead angle of engaged threads of the nut and the shaft portion is reduced, a rotational angle of the nut relative to the amount of linear displacement of the shaft portion is increased. In such a case, the linear drive force of the shaft portion becomes insufficient relative to the frictional resistance at the engaged threads of the nut and the shaft portion, so that self-locking occurs to limit the linear movement of the shaft portion. Thus, when the supply of the electric power to the electric motor is stopped, the reverse input from the controlled subject, which drives the shaft portion, is cut, and thereby a given position of the shaft portion can be maintained.
In order to effectively use the self-locking function in the actuator of JP4025155B2, an output transmission efficiency from the electric motor to the shaft portion needs to be reduced. Therefore, it is required to excessively largely set the motor output relative to the output of the shaft portion. Thus, a size of the actuator is disadvantageously increased.